There has been conventionally used a studless tire having relatively soft tread rubber as a tire for safely running on ice, as well as a dry road surface. It is known that on-ice performance of a tire improves when tread rubber of the tire is made relatively soft. However, a tire having soft tread rubber generally has a problem in that the tire exhibits poor wear resistance on a dry road surface. In short, on-ice performance and wear resistance of a tire cannot be improved at the same time in a compatible manner.
Examples of a known technique for improving on-ice performance of a tire include blending organic fibers, glass fibers or the like with a rubber composition for use in a tread of the tire so that the organic fibers, glass fibers or the like scratch an icy road surface and improve on-ice performance of the tire. However, such organic fibers, glass fibers or the like, exhibiting no interaction with rubber, rather function as starting points of breakage and tend to deteriorate fracture resistance (wear resistance) of tread rubber. It is therefore difficult to blend a satisfactory amount of organic fibers, glass fibers or the like with a rubber composition. That is, it is difficult to improve on-ice performance of a tire in a satisfactory manner by adding organic fibers, glass fibers or the like to tread rubber thereof.
PTL1 proposes as a tire for solving the aforementioned problems a rubber composition containing 0.5-20 parts by weight of potassium titanate fiber and 5-200 parts by weight of carbon black having 100-300 mg/g of iodine adsorption amount, with respect to 100 parts by weight of a rubber component comprising natural rubber and butadiene rubber. PTL 1 reports that use of the rubber composition in a cap tread layer of two-layered tread including cap tread and base tread improves performance on ice (and snow), while well suppressing deterioration of wear resistance of the tire.